1. Field of the Invention
The present invention relates to a fuel cell characteristic recovery method and apparatus, and particularly, a characteristic recovery method and apparatus for direct methanol fuel cells used in mobile and portable power supplies, electric automobile power supplies, home cogeneration systems, etc.
2. Description of the Related Art
From the point of view of global environmental protection, and the like, expectations for fuel cells have recently been rapidly raised. Fuel cells are generally classified according to kinds of electrolytes used into five kinds of solid oxide fuel cells (SOFCs), molten carbonate fuel cells (MCFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), polymer electrolyte fuel cells (PEFCs), etc.
Among others, polymer electrolyte fuel cells (hereinafter, “PEFCs”) with a polymer electrolyte membrane sandwiched between two electrodes, in which these members are further sandwiched between separators, are remarkable because of their compact structure, excellent power generation efficiency and relatively low-temperature operation, thereby having a wide range of applications.
Also, among PEFCs, particularly, direct methanol fuel cells (hereinafter, “DMFCs”) are recently remarkable, which, instead of a hydrogen gas, uses a methanol solution directly as fuel. DMFCs generate power by causing an electrochemical reaction of a fuel containing methanol and water, and an oxygen-containing oxidizer gas such as air. DMFCs have various application fields. For instance, because they operate at room temperature and can be made small and sealed, they can be used in pollution-free automobiles, home power generation systems, mobile communication equipment, medical equipment, etc.
A DMFC comprises, basically, as a unit cell (hereinafter, “cell”), a stacked body having conductive separators stacked on both sides of a membrane electrode assembly (hereinafter, “MEA”). The MEA consists of three layers in which an electrolyte membrane comprising an ion exchange resin or like is sandwiched between a pair of electrodes constituting anode and cathode electrodes. The pair of electrodes each consists of an electrode catalyst layer in contact with the electrolyte membrane, and an outer fuel or oxidizer gas diffusion layer (dispersion layer) of the electrode catalyst layer. The conductive separators are stacked so as to come into contact with the diffusion layer (dispersion layer) of the MEA, and are formed with manifold apertures which serve as passages for a fuel or oxidizer gas to flow into the diffusion layer (dispersion layer), separator temperature adjustment, waste removal, etc. Such fuel cells generate power by causing an electrochemical reaction, for example, when a mixture of methanol and water is caused to flow through the manifold apertures in contact with the diffusion layer (dispersion layer) of the anode electrode, while an oxidizing gas such as oxygen, air, or the like is caused to flow through the manifold apertures in contact with the diffusion layer (dispersion layer) of the cathode electrode.
In fuel cells including DMFCs, it is well known that continuing power generation would cause its power generation characteristic degradation, i.e., performance degradation such as a current density fall under a constant voltage.
Because such power generation characteristic degradation causes an obstacle in practical use of fuel cells, such as an increase in maintenance cost due to a short lifetime, there is a need for a method for rapidly and conveniently recovering the degraded power generation characteristic.
As a method for recovering the degraded power generation characteristic, there is known a method described in PCT Japanese Publication No. 2003-536232. PCT Japanese Publication No. 2003-536232 discloses a method for recovering the degraded performance of a proton exchange membrane (PEM) fuel cell by periodically reducing a cathode potential below approximately 0.6 V.
However, PCT Japanese Publication No. 2003-536232 only shows one method for recovering the degraded performance of a PEM fuel cell, so that there is a desire for a still more convenient and more widely applicable method.